Colorectal cancer is the third leading cause of cancer death in both males and females in the U.S. Thus, the development of dietary and novel chemopreventive approaches for this segment of the population could provide a practical approach for significantly reducing colon cancer occurrence. Recent studies indicate that supplemental Se (in the form of high selenium containing yeast) reduces cancer risk in humans. However, the mechanism by which organic selenium exerts its anticancer effects remains to be identified. Selenomethionine is the predominant form of selenium in the yeast. We have developed evidence indicating that selenomethionine, inhibits Cox 2 protein expression in colon cancer cells. Thus, the first hypothesis to be tested is that the anticancer effects of dietary selenium (in the form of Se-yeast or selenomethionine) are mediated, in part, by inhibition of cyclooxygenase 2 (COX-2) activity. We also have evidence for a COX-2 independent mechanism. We can demonstrate alterations in cyclin A, cyclin B, and p34 cdc2 phosphorylation in selenomethionine treated colon cancer cells. These observations lead to the second hypothesis of the proposed studies that selenomethionine causes growth arrest by inappropriate expression of cyclins A and cyclins B as well increased phosphorylation of p34cdc2. To test our two main hypotheses, the three specific aims are performed 1) To determine the molecular basis of the effects of selenomethionine on COX-2 protein expression. 2: To investigate the role prostanoid receptors play in mediating the downstream effects of PGE2 in selenomethionine-induced growth inhibition. 3: To identify the mechanism for the COX-2 independent anti-cancer effects of selenium. We have a unique opportunity to translate our basic science experimental findings to clinical intervention trials. The data from these studies will identify the key mechanisms involved in the anticancer effects of selenium and increase understanding of eicosinoids in the biology of colorectal cancer. The greatest impact is realized by the provision of new molecular markers for the efficacy of selenium compounds for clinical intervention trials and the development of novel molecular targets for therapeutic intervention strategies.